OpenSMTPD is an open-source server-side implementation of the Simple Mail Transfer Protocol(SMTP)that is part of the OpenBSD Project. OpenSMTPD’s smtp_mailaddr()function is responsible for validating sender and recipient mail addresses. If the local part of an address is invalid and the domain name is empty,smtp_mailaddr()will automatically add a domain name as opposed to failing because of the invalid local address. This will allow the invalid local address to pass through the function without validation.